An RFID (Radio Frequency IDentification) system, which has become popular in recent years, is used for article management, such as inventory management using RF tags attached to respective management target articles. Examples of such an RFID system are disclosed in Patent Literature 1 to 3. In the techniques disclosed in Patent Literature 1 to 3, RF tags are attached to respective management target articles, and the management target articles are managed in the following manner. That is, when the tag information of the RF tags can be read, it is determined that the management target articles are present, while when the tag information cannot be read, it is determined that the management target articles are not present. However, such an application of the RFID system has the following problems.
Firstly, there is a problem of unauthorized reading of tag information of RF tags. For example, in the case of using RF tags to manage commodities on store shelves in a retail store, a sales clerk who manages the commodities and consumers who plan to purchase the commodities can read the tag information of the RF tags which are attached to the respective commodities by a third party. In this case, for example, information on a commodity that is to be purchased or has already been purchased by a consumer can be linked to the consumer, which leads to a trouble of invasion of privacy. Further, when raw materials stored in warehouses and products to be shipped are managed by the RFID system, a third party can read tag information of RF tags, which are respectively attached to the raw materials and products, so that the storage/delivery status of the raw materials and products can be disclosed to the third party. This poses an information security trouble.
Secondly, there is a problem that the RF tags are costly. Currently, the cost of a UHF band tag has been reduced to less than about 10 yen. However, this cost is about two orders higher than the cost of a barcode that is used for article management, especially, commodity management. As a result, it is difficult in terms of cost to attach RF tags to respective articles for which the price is about 1000 yen or less.
A technique to solve such problems is disclosed in Patent Literature 4. Patent Literature 4 discloses a technique for managing articles in the same manner as in the method in which the RFID system is used. Specifically, Patent Literature 4 discloses a technique relating to a smart shelf that uses the RFID system and is adapted to monitor the presence or absence of an article on the shelf.
In Patent Literature 4, RF tags are arranged on a shelf. Articles are arranged such that each article to be managed (hereinafter referred to as “management target article”) prevents an RFID reader from reading a plurality of RF tags arranged on the shelf. In other words, in Patent Literature 4, the management target articles are arranged between the RF tags and an antenna attached to the RFID reader. Further, in Patent Literature 4, the quantity of articles is monitored by the following procedure:                (a) the RFID reader radiates electromagnetic waves to the shelf;        (b) the number of RF tags whose tag information cannot be read by the RFID reader due to the presence of an article is calculated; and        (c) the quantity of articles is calculated based on the information obtained in the process (b).        
Note that when the articles are arranged between the reader and the tags, the RF tags are adjusted such that the articles prevent the reader from reading the tags.
According to the above-mentioned technique disclosed in Patent Literature 4, when the management target articles are arranged between the RFID reader and the RF tags, that is, when the management target articles are present on the shelf, the articles block the RF tags and the RFID reader from being viewed, thereby preventing the RFID reader from reading the tag information of the RF tags. Specifically, when there is a management target article, the tag information of the RF tag corresponding to the article cannot be read, so that the presence of the management target article can be detected. On the other hand, when there is no management target article on the shelf, specifically, when there is no management target article between the RFID reader and the RF tags, there is no management target article that blocks the views of the RF tags and the RFID reader, so that the RFID reader can read the tag information of the RF tag. Thus, when there is no management target article, the tag information corresponding to the article can be read, so that the absence of the article can be detected. As a result, in Patent Literature 4, the presence or absence of articles can be detected and thus the articles on the shelf can be managed. It is based on the premise that the articles that can be managed contain metal, water, or the like, which prevents transmission of energy at radio frequencies.
According to the above-mentioned technique disclosed in Patent Literature 4, the RF tags are not attached to the respective management target articles and are left on the shelf. This avoids troubles such as information security troubles and invasion of privacy due to unauthorized reading of the tag information of the RF tags attached to the respective management target articles. According to the technique disclosed in Patent Literature 4, the first problem due to unauthorized reading of the tag information of the RF tags by a third party does not occur. Further, according to the technique disclosed in Patent Literature 4, the RF tags are not attached to the respective articles and are left on the shelf. Therefore, each RF tag can be repeatedly used, and thus the tag cost per article is substantially equal to a value obtained by dividing the price of a tag by the number of times the tag is used. That is, according to the technique disclosed in Patent Literature 4, the second problem that the RF tags are costly can be solved by using the RF tags a sufficient number of times.